1. Field of the Invention
The present invention relates to a rotation detecting device for detecting the angular displacement of a magnetic body and capable of improving the reliability of finishing the ends of a detecting coil.
2. Discussion of Background
The conventional magnetic rotation detecting device has a magnetic circuit comprising (1) a pole, (2) a bias magnet and (3) a detecting coil.
A high output voltage of the detecting coil can be obtained by means such as (a) providing a fixed pole having a high magnetic permeability, (b) providing a bias magnet having a high magnetic flux density or (c) increasing the number of turns of the detecting coil. Generally, the means (c) is employed to obtain a high output voltage of the detecting coil.
However, a very thin wire must inevitably be used for forming the detecting coil to increase the number of turns within a limited space and ordinarily a wire of 0.13 mm to 0.15 mm in diameter is used for forming such a detecting coil. The ends of the wire are connected to terminals for lead wires, for example, by soldering. If the junctions of the ends of the wire and the terminal is stressed locally, the coil may possibly be disconnected from the terminal or terminals.
Referring to FIGS. 5, 6(A) and 6(B) showing a conventional rotation detecting device, there are shown a pole 1 formed of a ferromagnetic material, a bias magnet 2 for applying a magnetic flux to the pole 1, a spacer 3 for enhancing the effective magnetic flux of the bias magnet 2, a bobbin 4 receiving the pole 1 therein, a detecting coil 5 wound on the bobbin 4, the ends 5a of a wire forming the detecting coil 5, which are respectively wound around the connecting sections 6a of terminals 6 provided on the bobbin 4, and lead wires 7 respectively connected to the lead connecting sections 6b of the terminals 6.
Projections 4a and 4b are provided respectively at predetermined positions on the outer circumference of the bobbin 4. The projection 4a is engaged with a cap 8, and the terminals 6 are fitted respectively on the projection 4b. The pole 1, the bias magnet 2 and the spacer 3 are fixed in place by the cap 8. The lead wires 7 are connected respectively to the terminals 6 by putting the cap 8 on the projection 4a. The bobbin 4 mounted with the detecting coil 5 is received in a housing 10 formed of a plastic material such as Nylon by injection molding. The housing 10 is attached to a transmission case or the like with a screw so that the rotation detecting device is disposed near a rotary member. The housing 10 has a cylindrical fitting part 10a, a reduced part 10b having a diameter smaller than that of the fitting part 10a, an annular groove 10c for receiving an O-ring 11 therein, and a supporting part 10d. A locking bush 12 is provided in the supporting part 10d of the housing 10 to prevent the screw fastening the housing 10 to a transmission case or the like from loosening.
When the rotation detecting device is mounted on the transmission case, the pole 1 is located near a projection provided on the rotary member. When the projection of the rotary member approaches the pole 1 as the rotary member rotates, the magnetic lines of force of the bias magnet 2 extend from the bias magnet 2 through the pole 1, the rotary member, the external space and the spacer 3 to the bias magnet 2. Since the magnetic lines of force extend across the coil 5, voltage is induced in the detecting coil 5 as the magnetic flux varies according to the variation of the distance between the projection of the rotary member and the pole 1. The rotating speed of the rotary member is calculated by using the induced voltage. Such an induced voltage induced in the coil of the rotation detecting device is used also for detecting the approach of an object.
In assembling this conventional rotation detecting device, the connection of the lead wires 7 and the terminals 6 and the connection of the ends 5a of the wire of the coil 5 and the terminals 6 are implemented in the following procedure.
(1) The lead wires 7 are fixedly connected to the lead wire connecting parts of the terminals 6 by crimping.
(2) The ends of the wires of the detecting coil 5 are wound several turns respectively around the U-shaped coil connecting parts 6a of the terminals 6, and then the ends of the wires of the detecting coil 5 are fixed to the coil connecting parts 6a by soldering.
(3) The lead wire connecting parts 6b of the terminals 6 are placed between the projections 4a and 4b of the bobbin 4.
(4) The cap 8 is put on the housing 10 so as to cover the terminals 6, the rear part of the bobbin 4 and the extremities of the lead wires 7.
When the rotation detecting device is thus assembled, a clearance .DELTA.x in the range of 0.2 mm to 0.3 mm is formed between the rear end of the lead wire connecting parts 6b of the terminals 6 and the projection 4a of the bobbin 4, and hence the terminals 6 are movable to the left, as viewed in FIG. 6(B), by a distance .DELTA.x when the lead wires 7 are pulled to the left. Therefore, the wire of the coil 5 may possibly be broken when the lead wires 7 are pulled to the left if the slack of the wire between the coil 5 and the coil connecting parts 6a of the terminals 6 is smaller than the clearance .DELTA.x. Such a trouble occurs before injection molding and will not occur after injection molding because the terminals 6 are fixed by the molding and the ends 5a of the wire of the coil 5 will not be pulled even if the lead wires 7 are pulled.
However, in placing the assembly of the bobbin 4, the coil 5, the terminals 6 and the lead wires 7 in a mold for injection molding, the lead wires 7, in general, are pulled to set the assembly accurately in the mold, which often causes the breakage of the ends 5a of the wire of the coil 5.